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OK, so I redid the friction rate calculation including the info you shared, here it goes again, what do you think now? Would you be willing to double check what I've included? Does it seem like I have too many fittings? Too few? If I end up with .05 friction rate, how bad is it? Do I need to increase of decrease the size of the ducts to compensate? Are there any settings on the furnace that might help? Thanks!

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Your furnace install manual should have a fan data chart(if you don't have one for your current furnace, the manufacturer might send you one if you email them).

It will give different CFM's at different static pressures. Knowing what your TEL is for your duct system, along with PD of devices. You can calc what static your system is operating at then. But you need the fan data. To know what CFM it can move at what static pressure(of course the anemometer will tell us what CFM is being moved also).

Check your ducts current size to the size it should be for your TEL and ASP. See what the difference is. On the FR chart.

EG: If your first piece should be 8X26, but is only 8X20. Then you know your system is running at a static higher then .50". Probably around 1.1"wc(based on both return and supply being equally undersizzed).

My guess is your running at a static pressure of over .35" in your current supply alone if your only moving 1200 CFM. If your moving more, then it is much higher.

For a new furnace install. If your current duct system isn't sized to .17" FR.
Then see what FR it is sized to for 1200 CFM. And then reverse calc to see what ASP you need. Then check the fan data chart for the new furnace to see if using a ESP of .6 or .7 will give you the 1200 CFM.

Or, if you have to alter the first several foot of trunk line(as in remove it and install a larger section). Sometimes, you can get away with relocating one or 2 supplies from the trunk line, and moving them to the plenum.

Or, just putting in 8 or so foot of new trunk line for the first few supplies.

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Wow, all I can say is I now know why good HVAC techs charge so much... and why the bad ones use "rules of thumb." This stuff is complicated!! Thanks for walking me through it, sorry I am so dense!

So, first of all, I am definitely going to get a new furnace. I really like the Goodman GMVC950704CX - 70,000 BTU 95% Efficiency, Two Stage Burner, Variable Speed Blower, Upflow/Horizontal Application Furnace and the price here seems quite good. Does this seem like a good choice? Do you have any issues with Goodman? Is this a good size for my situation?

I have prepared the CFM report based on the furnace putting out 960 CFM, but based on the manual, located here, depending on the furnace settings we can have between 756-1,188 CFM on the low stage heating setting, 1,089-1,711 cfm on the high stage heating setting, or 540-1,572 CFM on high stage cooling setting and 378-1,022. As I start to do the duct design, how do I know that total CFM to use? How do I calculate the optimal CFM for my home?

As far as the duct design goes, I would prefer to use as much of the existing ductwork as possible, BUT I am more concerned with having a well performing system and a comfortable home, even if I need to rip all of the existing ductwork out! So, pretending I have no ductwork at all, where do I start?

Tied into the support plenum I would like to have 4 trunks (is that the right word?), one for the basement, one for the upper level and two for the main level.

1) Once I figure out the optimal CFM for the home, I figure I will use a ductolator to figure out what size duct I need based on a FR of .05, right?

2) Can you suggest a ductolator? The one I have been using doesn't allow for a friction rate of .05. The lowest it allows is .06.

3)Also, after each supply line branches off from the trunk, the trunk should reduce in size, right?

4) How big of a problem is a .05 friction rate? It seems like there isn't much I can do about that specifically.

5) You mentioned static pressure in your last post, that hasn't come up before. What is its importance and what do I need to do?

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The price looks ok for the furnace.

CFM for duct sizing is based on the equipment/mode that needs the most air. Heating or cooling.
A 70,000BTU input 95% furnace needs to move 1120CFM to have a 55°F temp rise through it(That Goodman is probably rated for a temp rise between 40 and 70, best to pick middle or lower end, allows for air filter to get dirty that way).
I believe that model's lowest CFM for high heat is 1210, minus 10% for comfort, for a lowest setting of 1080CFM in high heat. So you know that your duct system needs to be able to move that as a min for heating.

The A/C, at 3 tons as rule of thumb would be 400 CFM per ton, or 1200 CFM.
In reality. The A/C CFM should be based on the SHR(sensible heat ratio) that your house needs. Meaning it might only need 1100, or a 1000CFM of air flow.

I believe your homes SHR was .79, so your A/C(if the unit is the size the house needs) CFM can be set to as high as 435CFM per ton to achieve that.
Since its a 3 ton though. You would want to adjust that for that oversize. And reduce it to 365CFM per ton. Giving you a total A/C CFM requirement of 1095CFM. Making your heating CFM your designing CFM.

Starting from a no duct work view.

You start with drawing out your trunk lines, and supply branches. Just as simple line drawings. With the linear length written at them. And then after all trinks and branches are drawn and listed. You go back and figure out their TEL to determine which one is the longest run. And then apply the ASP math formula from the previous post. And then you would look up the duct sizes you need at that FR.

I get my ductulators from the supply houses. You can order one from several different sites on the net. ACCA is one of the sites. I don't know the rest cause I don't use them.

The trunk line doesn't need to reduce at all. If its less then 28' long.

On a reducing trunk. You reduce when your velocity decreases/would decrease too much. Too much is the part that can be hard to determine. Or you can reduce to maintain velocity to a preferred velocity(preferred just means that is what you want, even though the system doesn't need to maintain that velocity).

EG: 17"(10X24) round at 1200 CFM is a velocity of 762FPM, when you have 2 take offs using up 200 CFM of that, you reduce to 15.5"(10X20) to have 763FPM.
The small/short distance between the 2 supplies is often only 1 to 3 foot, and you don't want to reduce every foot or so since that creates additional turbulence in the duct that interferes with air flow.

Static pressure is part of ESP. External Static Pressure. In the case of a gas/oil furnace. its all the devices we have been talking about, including the A/C coil, because the A/C is not pert of the furnace. The evap coil is External to the furnace.

Static pressure that you are probably familiar with. Blow up a balloon. The amount of force it takes you to do it, is the force you must exert against the static pressure of the balloon material.

Static pressure is what the blower works against to move air. Its the force exerted in all directions in the duct, when the blower is running.
So we want to keep that low, so the blower can move the amount of air we need/want.
And on a VS blower. If that gets high, the blower uses more electric to move the air.

.05" FR is more in line with making your duct system as easy as possible for the blower to move the air through it.
How ever, if you don't want to buy a ductulator. Using .06" won't kill the system.

You can use 4 trunk lines if you have the room. Using 4 trunk lines may or may not reduce TEL and allow you to use a higher FR and still maintain a low ESP. You would need to recalc after you make a drawing of your duct system.

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Jim-

Sorry it has taken me so long to say thank you. Your last post was incredibly instructive. I've learned so much, it really is great. I am attaching a copy of the latest CFM report based on 1080 cfm based on what you indicated in your last post.

I drew the ductwork out this weekend, just to double check my TEL. Unfortunately it was correct, so I am stuck with the .05 FR. Is this OK?

I have a couple of hopefully basic questions now as I am figuring out duct sizes.

1) Is it helpful to place a supply register in a hallway?

2) The ductolator I am using doesn't allow for a FR under .06. By using this, will it end up oversizing or undersizing the ducts? Is there anything I can do to account for that? Is there enough of a difference for me to try and pickup a ductolator?

3) I am trying to figure out how much of my existing ductwork I can use. The ductolator Currently I have a 12x10 and a 12x8 running to the attic to service the second floor, the 12x10 as a supply, the 12x8 as a return. The CFM report shows that the second floor needs 575 CFM. According to the ductolator, the 12x10 will move 575 CFM at 690 FPM. Is that OK? The 12x8 will move 575 CFM at 863FPM? I know that is high, but will that work? If the existing 12x8 and 12x8 can work, which is better for return and which is better for supply?

4) On the other side of the same question, is 398 fpm or 328 fpm too low for a trunk? We need to move 221 cfm and 182 cfm and currently there are 8x10 trunks for both.

5) You said 4" supply lines are OK for bathrooms, but that all other rooms, regardless of their demand, should have 5" or more. Did I understand you correctly? So, even if a room only needs 14 cfm, I should still use a 5" line and then just use the register dampers, right?

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Your welcome.

The new furnace will be able to operate fine even if its working against a starting ESP of .7"(starting FR referring to a new clean air filter). So using .06 (or .07" max FR) will work ok( above .07", you'll end up with a starting ESP of over .82", which will mean a VS blower will start to use just as much electric as a standard blower after only a week or 2).

Registers generally aren't needed in a hallway, as long as it walls are all inside walls, and the rooms it serves use it as a return air path.

Although the velocity for 575CFM through the 12X10, and 12X8 would be ok. What will happen, is that the static pressure in those 2 different sizes will prevent them from having the same amount of air flow.

EG: 575CFM through a 12X10 duct is a FR of .072", 575 CFM through a 12X8 duct is a FR of .13"wc.
So in this case. The return is too restrictive to return the 575CFM from the second floor.
So the solution would be to add another return some where to make up the additional 160 CFM that the 12X8 won't move at the same FR as the 12X10 supply.

If the 10X8 is a long run, or goes through unconditioned space, then 330FPM is a bit too low/slow.
If its a short trunk line run, then its no problem.

Your report shows 11CFM for the guest bedroom. While it might be tempting to use a 4" for it. I think you'll find that under operating conditions, that room will need double what the report says.
Reason, is it doesn't allow for 2 people to be in the room with that 11 CFM(2 people are a sensible cooling load of 400 BTU's). So you need to be able to deliver at least twice as much air, if you have company staying over for more then one night during the summer( sometimes 3 times as much, if they are "active" people, ). Along with the possibility that the room could become a computer, or office room some where down the road. So I wouldn't run less then a 5" to a bedroom.

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Excellent information. This is really coming together... at least in my mind. I managed to sell my wife on the new furnace, but tearing up the basement is going to be a harder sell (her mom lives there)!! Oh well, all for the good of the order, as they say.

I have a couple of follow on questions...

1) The CFM report calls for about 30 CFM in the upper level hallway. All of the walls are interior, but the ceiling is somewhat under insulated. Should I just divide that amount up amongst the other rooms?

2)Currently I have a single return air register in the upstairs hallway and all the doors are undercut, but I have been considering adding return air vents to each room. Is this worth the effort?

3) So, it seems like I can keep my existing 12x10 and 12x8 if I add an additional duct. This shouldn't be too difficult. It looks like the 12x10 @ 575CFM has a FPM of 690 and the 12x8 @ 460CFM has a FPM of 690, so I need to account for an additional 115 CFM, does that sound right? How did you come up with 160CFM? The ductolator shows I can use a 5.5" round (that doesn't exist, does it?) with 697 FPM or a 5x5(662) or a 8x3 (690 FPM). So, would something like this work?

4) With the distance of the 10x8, are you referring to its actual length, or its effective length? The actual length is approximately 20', but it connects into the side of the plenum, so with the take off, it is 100'. Can this work, or do I need to change it? The other 10x8 serves the other half of the main floor, is 13' actual or 93' effective. Would be great if I could make this work, but I want it to work correctly more than anything else.

5) Just FYI, the guest bedroom is a bit misleading. I am attaching a copy of the floor plan. The addition we are gearing up for will add a small annex to the bedroom which will connect to the bedroom via a "doorway," but there will not be a door. The CFM report calls for 80 CFM in the "annex," largely because it has 2 exterior walls, and just 11 CFM in the bedroom itself. How do you think I should break this up? Did the CFM report get it right, or should I make some changes to it?

6) Reasonably, what is the max amount of CFM you want coming out of a single register (floor or ceiling) before you break it up into 2 registers?

7) Also, I am a little confused on the CFM report. Take for example the upper level of the home, the CFM indicates total CFM of 575, but when you had up the totals for each room it comes out to 654 CFM. Which should I size the trunk for 575 or 654?

8)This is more of a logistical question, but how big of a round duct can you tie into the 6" side of a 6x8 rectangular duct? Can you tie in a 6" round? Or just a 5"?

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Quote:

Originally Posted by stubits

Excellent information. This is really coming together... at least in my mind. I managed to sell my wife on the new furnace, but tearing up the basement is going to be a harder sell (her mom lives there)!! Oh well, all for the good of the order, as they say.

I have a couple of follow on questions...

1) The CFM report calls for about 30 CFM in the upper level hallway. All of the walls are interior, but the ceiling is somewhat under insulated. Should I just divide that amount up amongst the other rooms?

Yes, if the hallway is part of the return path the air from those rooms will take.

2)Currently I have a single return air register in the upstairs hallway and all the doors are undercut, but I have been considering adding return air vents to each room. Is this worth the effort?

It can be, if those rooms don't maintain temp when the bedrooms doors are shut at night. If they do maintain temp with the doors shut. It probably isn't worth while.

3) So, it seems like I can keep my existing 12x10 and 12x8 if I add an additional duct. This shouldn't be too difficult. It looks like the 12x10 @ 575CFM has a FPM of 690 and the 12x8 @ 460CFM has a FPM of 690, so I need to account for an additional 115 CFM, does that sound right? How did you come up with 160CFM? The ductolator shows I can use a 5.5" round (that doesn't exist, does it?) with 697 FPM or a 5x5(662) or a 8x3 (690 FPM). So, would something like this work?

Your still concentrating on velocity too much. Look at the friction rate of a 8X12 moving 460CFM. Its better to match FR, then velocity.
You'll get about 415CFM at the saame FR (.072")as the 10X12. So its better to use the lower FR that the 12X10 is at, then to increase the FR of the 12X8.

The return system has to be looked at as a whole. if you increase resistance to air flow in one return, the other returns will now draw in more air then you planed. Because that return is more resistance to air flow.

4) With the distance of the 10x8, are you referring to its actual length, or its effective length? The actual length is approximately 20', but it connects into the side of the plenum, so with the take off, it is 100'. Can this work, or do I need to change it? The other 10x8 serves the other half of the main floor, is 13' actual or 93' effective. Would be great if I could make this work, but I want it to work correctly more than anything else.

The actual distance. A 7' difference shouldn't be a problem though.

5) Just FYI, the guest bedroom is a bit misleading. I am attaching a copy of the floor plan. The addition we are gearing up for will add a small annex to the bedroom which will connect to the bedroom via a "doorway," but there will not be a door. The CFM report calls for 80 CFM in the "annex," largely because it has 2 exterior walls, and just 11 CFM in the bedroom itself. How do you think I should break this up? Did the CFM report get it right, or should I make some changes to it?

If the return for the annex has to go through the bedroom. Then having more supply in it works fine. If the annex will have a return, then the bedroom should have the most supply of the 2.

6) Reasonably, what is the max amount of CFM you want coming out of a single register (floor or ceiling) before you break it up into 2 registers?

That depends on where the register is. A high sidewall can have mofre air flow then a floor register. Since it won't be blowing air on anyone. On a floor register, 120 is about the max. With the exception being. that sometimes its now practical to split up a 140 CFM supply. So in that case, a 4X14 at 140CFM is ok. Anything above that is too much air to be able to stand anywhere near it, and be comfortable.

7) Also, I am a little confused on the CFM report. Take for example the upper level of the home, the CFM indicates total CFM of 575, but when you had up the totals for each room it comes out to 654 CFM. Which should I size the trunk for 575 or 654?

I believe its a flaw in the program. Its not using the higher CFM of the master bedroom in heating, as the both CFM calculation.

8)This is more of a logistical question, but how big of a round duct can you tie into the 6" side of a 6x8 rectangular duct? Can you tie in a 6" round? Or just a 5"?

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As always, thanks so much... I am sorry for all the questions, but without your help I'd be totally lost!! Thanks so much!

-Maybe I will try the central return on the upper level and if I don't get the results I want, I will change it for individual returns. All the doors are undercut AND we rarely shut the doors, basically only when we have guests, so not often.

-OK, so I have been ignoring FR because the ductulator I've been using doesn't give that, just CFM and velocity. I am guessing your ductulator also lists this information. I should head over to the supply house and pick one up, right? Or is there a way to figure out FR for each run of duct based on the info I have (CFM, velocity, distance, etc.)? Is FR also equal to "head loss"? I found a new ductulator here which seems a lot more complete.

- Regarding my previous question #4, I just want to confirm, given the relatively short distances, 20' and 13', it will be OK to use the two existing 8x10 trunks even though the velocities are 398 fpm for 221 cfm and 328 fpm for 182 cfm. Will this work?

- Referring to the two trunks above, which each serve half of the main level, and will share a single return vent, do I need to balance them in some way?

- Currently one of our floor vents is just a hole cut in the duct with a register placed over it. There is no take off and no register boot. Does this work at all or is it totally wrong?

- Regarding the bedroom annex, yes, the return air will have to pass from the annex through the bedroom to get to the return. That said, will it be more comfortable to have more supply in the bedroom? This will be new ductwork, so I can do whatever I want here.

-OK, so no more than 120, maybe 140 out of a floor register... how about a ceiling register? What is the max comfortable there?

Thanks... and I am serious about owing you a beer or your drink of choice, or heck, even a steak the next time I am up in Lancaster!

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Yes, they are using the term head, in place of FR.
Thats a good program.

On the 2-8X10 trunks. You will probably have to balance them out. Some dampers in the supply runs themselves, near the take offs will take care of that.

As long as the annex doesn't have a door. The annex should get the air that is listed in the report.
Since the annex returns through the bedroom, there should be no problem heating or cooling the bedroom.

Should be a sealed path for the air to travel to that floor register. With just a hole in the floor and duct. Your probably getting a fair amount of air, blowing out around the register.

Ceiling register can have more. But it varies with the height of the ceiling for noise considerations.
But on the average house. If you follow the same limits as a floor register, you have a quiet system. With no harsh air currents.

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I will work today on figuring out the optimal duct sizes vs. what I have, along with velocity AND FR/head loss. It feels like we're getting close!!

Can you help me to understand FR a bit more? 1) Is it important to have a uniform FR throughout the entire system? 2) Or, should the FR just be uniform on each floor? In a "perfect" system, what should I be aiming for?

Oh and in the new McQuay ductulator, what should I use as the "air" setting? Not sure what else to call it, but it is the first drop down box. Is 68 degrees STP the best choice?

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Friction rate, is the amount of pressure that will be lost in the duct moving that amount of air.
Its the resistance to air flow that the duct material's surface places against air moving in it.

So its the rate/amount of pressure that will be lost to the friction of the duct surface area. No matter what the starting pressure is.

If you start out with .4" pressure, you will lose .1"wc pressure per 100 foot of TEL of 6" pipe per 110 CFM moving through that pipe.

If you start out with .9" pressure, you will lose .1"wc pressure per 100 foot of TEL of 6" pipe per 110 CFM moving through that pipe.

The pressure loss/friction rate is always determined by the amount of air moving, not the starting/entering pressure.

The higher the FR, the more pressure it takes to move the air that is touching the inside surface area of the duct.

So it requires more pressure from the blower. But blowers aren't made to work against a high pressure. So as the pressure in the duct increases. The amount of air the blower can move decreases. Thats why its important to keep the Total External Static Pressure low.

Take a 5/8"ID garden hose and a 3/4"ID garden hose. Which one will flow more water at a entering pressure of 40PSIG?
The larger one, since the area inside of it is bigger, its less resistive to water flow.

How much more pressure would you need to get the same amount of water from the smaller hose(I didn't bother to look that one up)?

Air is the same way. Larger duct has less resistance, so it takes less pressure to move air through it.

Its best to maintain, or to try to maintain the same FR(not always possible though).

In a perfect world, on the perfect system. Each supply and return would have its FR set for that individual run's TEL.

Use the 68STD setting. The difference between that and the air that your furnace will be moving in either cooling or heating mode is too small to worry about. Usually less then .005"wc. Five one thousandths of an inch is not worth making the correction for on a system that will vary its air flow from heating to cooling mode.